1. Field of the Invention
The present invention relates in general to a polishing apparatus for producing a flat and mirror polished surface on objects such as semiconductor wafers using a grinding plate fixed to a base section and a method of polishing using such a grinding plate.
2. Description of the Related Art
As the density of circuit integration in semiconductor devices becomes ever higher, circuit patterns are becoming finer and interline spacing is becoming narrower especially for patterns of extreme fine line width. Accompanying this trend, the depth of focus becomes very shallow in photolithography of circuit patterns, and it requires that the surface of the wafer placed at the focal plane of a stepper must be microscopically flat to produce the required degree of image sharpness. A method of obtaining a flat surface is to polish the object in a polishing apparatus.
A type of conventional polishing apparatus is comprised by a turntable having a polishing cloth and a top ring which presses and rotates against the turntable with a given pressure while the polishing object is held therebetween and supplying a polishing solution to the interface to produce a flat and mirror polished surface.
FIG. 1 shows the essential configuration of an example of a conventional polishing apparatus. The apparatus is comprised by a rotating turntable 5 having a polishing cloth 6 such as a urethane cloth 6; a top ring 1 holding an object 4 such as a semiconductor wafer, so as to press and rotate the object against the cloth 6; a spray nozzle 9 for supplying a polishing solution Q to the cloth 6. The top ring 1 is connected to a top ring shaft 8, and the top ring 1 holds a wafer 4 in contact with an elastic mat 2 such as polyurethane. The top ring 1 has a cylindrical guide ring 3 on its outer periphery so that the wafer 4 would not be disengaged from the bottom of the top ring 1. The guide ring 3 is fixed in position relative to the top ring 1, and the wafer 4 is held within the holding surface so that the wafer 4 would not jump outside of the top ring due to frictional forces with the cloth 6. The top ring 1 is supported on a spherical bearing 7 so that it can be tilted with respect to the shaft 8.
While holding the wafer 4 in the bottom surface of the elastic mat 2 of the top ring 1, wafer 4 is pressed against the cloth 6 on the turntable 5, and the turntable 5 and the top ring 1 are independently rotated so as to slide the wafer 4 relative to the cloth 6 to polish the wafer 4. In this case, polishing solution Q is supplied from the nozzle 9 to the top surface of the cloth 6. The polishing solution combines two effects of chemical and mechanical polishing (CMP), namely, chemical polishing by an alkaline solution, for example, and mechanical polishing using abrasive particles, for example silica particles, suspended in a solution.
However, the conventional polishing methods of CMP using such a slurry solution containing many abrasive particles presents two operational problems.
The first problem is that, depending on the pattern types and the difference in the elevation of the surface structures, a polished surface can result in surface waviness or undulations. In general, circuit patterns formed on a semiconductor wafer are comprised by various patterns having differing dimensions and elevations. These surface structures can include micro-structures of separation distance of the order of micrometers and height of 0.5-1 .mu.m, and macro-structures of separation distance of the order of 100 .mu.m to 1 mm. Surface films such as aluminum silicates and aluminum films are deposited on the wafers having such elevation differences, and when such a surface, having surface elevations polished, the polished surface will reproduce the base elevations. When such microscopically uneven surface is subjected to a CMP process, raised portions are removed together with the depressed portions of the surface, leading to a creation of surface undulations. It is considered that this phenomenon in CMP is created because a relatively soft cloth such as urethane is used along with abrasive particles of microscopic sizes contained in the polishing solution so that not only the raised portions but the depressed portion of the surface structure is also removed by such abrasive particles.
The second problem relates to cost and environmental considerations. The polishing solution is usually a slurry containing a fine silica or the like powder in suspension, but to obtain a uniformly flat surface of high quality, it is necessary to supply the solution in an abundant quantity to the polishing surface. However, most of the solution supplied is actually discharged in the spent solution without contributing much to the flattening process. Polishing solutions used for high precision polishing of semiconductor devices are expensive, which is a factor leading to a problem of high polishing cost. Also, because such polishing solution in a slurry condition contains a large quantity of silica or the like particles, the maintenance of the environment becomes difficult. That is, contamination of a solution supply system and drainage system by the slurry is serious, and the spent solution must also be treated extensively before being discarded. Also, after a CMP process, the wafers are washed to remove the polishing solution, but the waste water from this operation also must be treated in a similar manner and poses an environmental problem.
Therefore, instead of using a slurry on the polishing cloth, a method of using a grinding plate has been known. This method is based on a grinding plate comprised by silica or other abrasive particles bound in a flat ground platen, which is fixed on top of the polishing table. The wafer is polished while being pressed between the top ring and the grinding plate.
According to this method of polishing, because the grinding plate is harder than a cloth, only the raised portions of the surface are polished, thereby eliminating the problem of creating surface undulations described above. This means that precision polishing is possible, that is, to polish only those areas needed to be processed. Also, because a slurry containing a large amount of particles is not used, the processing cost can be low, and the environmental load can also be decreased.